I would think the web UI would be best. Log into your exchange online and archive there with multiple selections.
Spark also has a guide on using their app to do this. However, when you are dealing with thousands of emails it is better to do them in groups of 100. Use search and filter to purge things.
Louis L'Amour, “To make democracy work, we must be a nation of participants, not simply observers.
One who does not vote has no right to complain.”
Visit our archived Minecraft world! | Maybe someday I'll proof read, until then deal with it.